Angle sensor and a steering system based thereon

ABSTRACT

The phenomenon of light polarization is exploited to provide an improved angle position sensor having a high speed of response, negligible wear, and high resolution. The sensor has first and second parts movable relative to each other and incorporates a light source, a photosensitive element such as a photocell, and polarizing means disposed in the path of light passing from the light source to the photosensitive element, the signal from the photosensitive element giving a measure of the angle of rotation between the parts. Means for imparting a saturation-type response may be used in conjunction with the sensor with a view to avoiding problems connected with possible ambiguity of angle quadrant. The sensor lends itself to control applications and navigational devices.

United States Patent 1 Flory et al.

154] ANGLE SENSOR AND A STEERING SYSTEM BASED THEREON PrimaryExaminerSamuel Feinberg [75] Inventors: Brian Allen Flory; Roger Michaelj i m iqfi; Buczmskl Geldard, both of Kent, England [73] Assignee: Sharpand Company Limited, Kent, [57] ABSTRACT ng and The phenomenon of lightpolarization is exploited to [22] Filed: Oct. 1, 1969 provide animproved angle position sensor having a hi h s eed of res onse ne liible wear and hi h 21 A LN .1 862,737 g P P gg g 1 PP 0 resolution. Thesensor has first and second parts movable relative to each other andincorporates a [52] U.S. Cl. ..356/152,250/225,250/231SE, li ht Source,a hoto n itive element such as a 250/2 318/583 photocell, and polarizingmeans disposed in the path [51] Int. Cl. ..G05d 1/00 f li ht assin fromthe light source to the photosenl l Fmld oiSe1llch-356/117, 250/231 255,sitive element, the signal from the photosensitive ele- 250/204;318/588ment giving a measure of the angle of rotation between the parts. Meansfor imparting a saturation- References Cited type response may be usedin conjunction with the UNITED STATES PATENTS sensor w th a view toavoiding problems connected with possible ambiguity of angle quadrant.The sensor 2,862,169 11/1958 Yragui 318/588 lends itself to controlapplications and navigational 2,059,271 ll/l936 Parker ...3l8/588devices. 2,565,213 8/1951 Falkenstein.. ...250/225 3,478,209 11/1969Feuer ..2SO/7l 7 Claims, 4 Drawing Figures r lA I 5/! l I A I I 1 1 Mar.27, 1973 PATENTEDmzmn 3 3,01 1

INVENTOR BRIAN ALLEN FLORY ET AL ATTORNEY PATENTEDHARZYISH 3 7 ,011

SHEET 2 OF 3 Rlg TOLOAD TO LOAD 5 2 INVENTOR BRIAN ALLEN FLORY ET AL BYZ ATTORNEY PATENTEDHAR27I975 SHEET 3 BF 3 INVENTOR BRIAN ALLEN FLORY ETAL ATTQRNEY ANGLE SENSOR AND A STEERING SYSTEM BASED THEREON The presentinvention relates to position sensors suitable for control systems, moreparticularly to sensors responding to the angular position of a member.

As is well known in the art, control systems and regulators veryfrequently involve the determination and comparison of the angularpositions of-a control (input) shaft and a driven (output) shaft toproduce an error signal which is arranged to act so as to bring thedriven shaft into the required relationship with the con trol shaft. Theangular position of a shaft is very often determined with the aid of anelectrical potentiometer connected or geared to the shaft in somemanner.

Potentiometer angle sensors of this kind have a number of disadvantages,including a comparatively high frictional resistance to rotation,significant wear and consequently limited service life and, in the caseof wire-wound potentiometers, a limited resolution. In one of itsaspects the present invention, which utilizes an optically polarizingmedia, is the outcome of efforts to provide a solution to theseproblems.

When a beam of unpolarized light is passed through an opticallypolarizing medium, such as polaroid sheet, for example, the output beamis substantially planepolarized. As is well known, it is possible todefine a direction (line) in such a medium such that if an incident waveis plane-polarized parallel to that direction then the wave istransmitted with minimum attenuation, whereas if it is plane-polarizedperpendicular to that direction the attenuation is at its greatest. The

' transmission through two superimposed sheets crossed at right anglesis theoretically always zero.

According to one aspect, the invention provides a angular positionsensor having first and second parts rotatable relative to each otherand comprises a photosensitive element, a light source for illuminatingthe photosensitive element, and light-polarizing means disposed in thepath of light passing from the light source to the photosensitiveelement, the signal from the photosensitive element giving a measure ofthe magnitude and sign of the angle of rotation between the parts.

The polarizing means may be entirely, or partly, combined with thesource and/or photosensitive element, e.g., a polarized source and/orpolarization-sensitive photosensitive element may be used.Alternatively, successive polarizing elements, e.g., Polaroid" film, maybe interposed between the source or sources and the photosensitiveelement or elements.

In any event, the polarizing arrangement is such that light from thesource is polarized by a first polarizing means and is detected to yieldthe required angular measure via further polarizing means, the amount ofcrossing of the polarizing means being arranged to depend on the angularposition to be sensed so that the output of the photosensitive elementor elements varies with the angular position. To obtain an indication ofthe sign of the variation of the angular position, at least twophotosensitive elements are desirable; an asymmetrical polarizing systemwould give a direction-sensitive output from a single element, but thepossible range of angular variation would be limited. We prefer toprovide two photosensitive elements, and for each a light path includingrelatively rotatable polarizing means,

the respective polarizing means in the two paths being arranged so thattheir degrees of crossing (i.e., the obscuration of light produced bythem in the paths) vary in opposite directions upon rotation relativelyto each other, so that the outputs of the photosensitive elements give ameasure of the sign and magnitude of the angle of rotation. Thisarrangement is particularly ap plicable to sensing the course of a shipor other vehicle, each light path including a polarizer fixed to thevehicle and a polarizer connected to a compass, so that the outputs ofthe photosensitive elements indicate the deviation of the vehicle from achosen heading. In general, the light source and photosensitive elementsare fixed to the vehicle.

In one preferred constructional embodiment, we provide an angularposition sensor including: a polarizing arrangement comprising apolarizing card and a segment structure made of two polarizing segmentsrigidly connected relative to each other and arranged so that theirlines of polarization are non-parallel and preferably orthogonal, thepolarizing sheet and the segment structure being superimposed one on topof the other and rotatable relative to each other about an axis; twophotosensitive pickups arranged on one side of the polarizingarrangement; and a light source arranged on the other side of thepolarizing arrangement so that during the relative rotation of thepolarizing card and the segment structure, signals developed by thepickups, as a result of the varying intensity of the light transmittedto them from the light source through the polarizing arrangement, give ameasure in magnitude and sign of the angle of the relative rotation.

The two polarizing segments may take the form of quadrants of circles,while the maximum angle of relative rotation may be limited to i45 withthe aid of stops to give a saturation as opposed to an oscillatory typeof response, thereby preventing loss of sense for demands greater than:45" in the case of control systems.

To permit long life combined with minimum attention and maintenance, thelight source or sources may conveniently be nuclear-powered lightsources.

The angle position sensor lends itself to the construction of steeringand blind navigation devices for vessels and moving objects in general.The segment structure can be clamped to the vessel or object inaccordance with a cource to be steered, while a compass magnet isrigidly fixed to the polarizing card, which is free to rotate about anaxis due to the torque exerted on it by the magnet in the Earthsmagnetic field. The magnet and, consequently, the polarizing sheet alignin the Earths magnetic field, i.e., point North. The magnet mayadvantageously be a ring magnet concentrically disposed relative to theaxis of rotation of the polarizing sheet.

The clamping, i.e., the orientation of the segment structure relative tothe vessel, may be varied in accordance with a preset program so that arequired and, in general, complex course may be followed. Theprogramming device could be such as to permit dialing of a direction. Inthe simplest instance the program would provide for a sequence ofstraight-line segments to be traversed at constant velocity.

The programmed course may be followed automatically or manually. In thelatter case audio or visual means are provided which produce a signalwhen the guiding arrangement is brought into a new orientation by theprogramming device; the person or object carrying the guidingarrangement is thereupon required to turn until the null position (i.e.,no signal from indicator) is reached, which will correspond to the newcourse.

A signal derived from the photo-sensitive pickups gives a measure of themagnitude and sign of the deviation off course.

If the steering is to be automatic this signal is fed to a comparator,amplified, and employed to drive a means for turning the rudder in sucha manner as to correct the deviation. In the normal manner of automaticcontrol systems a signal is derived from the angle of rotation of therudder and likewise fed to the comparator, the difference signal outputfrom which constituting the loop error signal. The rudder positionsignal may, of course, be derived with the aid of a sensor according toour invention.

A control system employing a relay switching circuit for reducing theerror signal to zero is disclosed in our British Pat. No. 869,590.

If the steering is not required to be automatic the signal from thepickups, in the event of a deviation off course, can be employed toactuate an indicator device as aforesaid to suitably instruct thehelmsman. Such a device may be a buzzer arrangement, for example, thenote from which (high or low) would instruct the helmsman whethersteering to port or starboard is necessary in order to correct thedeviation.

A more simple arrangement utilizing only a single photosensitive pickupcan be employed to produce a signal indicating, without sign, thepresence of a deviation off course. Such an arrangement can clearly playthe role of an off course alarm.

A known steering device exists in which the shadow from an (ordinary,i.e., non-polarizing) card attached to a compass magnet is employed toproduce, as it passes over the input windows of two relatively fixedphoto-sensitive pickups, a signal giving a measure, in magnitude andsign, of the deviation off course. This known system requires verycareful construction if it is to operate at all well, its response tendsto be nonlinear, it suffers from shadow parallax effects due to compasstipping, and its response curve saturates at typically around :Ll, whenthe pickup windows become completely covered by shadow. The anglelimitation of il0 slows down the speed of automatic steering veryconsiderably, as the rudder deflection is normally set to be a givenfraction f of the demanded steering, which can in effect never exceedaround il0 in the known arrangement.

Our arrangement is free from all the above shortcomings and has,moreover, a response which is substantially linear over a range of 145,and which may thereafter be made to saturate, e.g., with the aid ofstops, in order to preserve the sense of the signal for demands greaterthan i45.

The invention will be further described with reference to theaccompanying drawings, in which:

FIG. 1 shows a schematic side view of an embodiment of a sensoraccording to the invention;

FIG. 2 shows a schematic plan view of a polarization arrangement (indirection AA in FIG. 1

FIG. 3 is a circuit diagram of a semiconductor differential switch forproducing a loop error signal; and

FIG. 4 is a schematic diagram of a system for producing a loop errorsignal with provision for audio or visual indication of error sign.

In FIGS. 1 and 2 reference 1 denotes a polarizing card and references 2,3 polarizing segments, shaped as quadrants of circles in the embodimentshown. The card and polarizing segments are made from polarized sheetmaterial. Card 1 is pivotally supported, as by a pin 4 in thisembodiment, for rotation about an axis 10, segments 2, 3 are fixedrelative to the casing 5, which is provided with indentations tofacilitate mounting of a light source 6 and photo-sensitive pickups 7,8. Casing 5 may advantageously be filled with liquid, and an air spaceor chamber 9 may be provided to take up any expansion of the liquid.

For applications to steering systems card 1 is provided with a rigidlyattached mag-net, which may be a ring magnet as shown at 11 in FIG. 1.

In FIG. 2 the card is shown as basically circular in shape with arcuateportions or slot removed so as to subtend an angle of at the center.Stops 12, 13 fixed to the casing are provided which can come up againstthe radial edges of the arcuate portions to limit the rotation of cardto i45 and thereby preserve the sense of the signal for desiredrotations greater than i45 (and less than To facilitate theunderstanding of the operation of the device, the direction ofpolarization for each piece of polaroid material is shown by lines (seeFIG. 2). In the mean position shown in FIG. 2, photosensitive pickups 7,8 will receive equal light intensity and their outputs will be the same(the mean level). If card 1 is rotated clockwise the light intensityreaching photosensitive pickup 8 will fall and that reachingphotosensitive pickup 7 will rise by an amount depending on the angle ofrotation, the electrical signals from the photosensitive pickups 7, 8varying appropriately about the mean level. If card 1 is rotatedanticlockwise the opposite situation obtains, so that the signals fromthe photosensitive pickups 7, 8 give a measure of the magnitude and signof the angle of rotation.

FIG. 3 shows a semiconductor differential switch suitable for thereduction of a loop error signal to zero. It may replace the relayswitching system disclosed in our British Pat. No. 869,590 mentionedabove.

The circuit comprises two antiparallel transistors or the like connectedbetween signal inputs and having bias arrangements such that, dependingon the relative signs and/or magnitudes of the input signals, either oneor the other, or neither transistor conducts; in particular if the inputsignals are equal neither transistor conducts, if one or the othersignals is greater than the other transistor conducts. In one possiblearrangement the emitter-base circuits and respective base resistors areantiparallel and the collectors form respective outputs, e.g.,controlling output stage transistors.

The circuit forms a three-state differential switch, i.e., it will givethree different states of output from a differential amplifier. Thestates are governed by the balance or unbalance of the differentialamplifier, and by The polarity of the unbalance. This means that with anunbalance in one direction a transistor, with its emitter tied to rail,will be turned on. While an unbalance in the other direction will turnthis transistor off and another on. When the differential amplifier isbalanced then both transistors are off. The circuit has the safeguardthat it is impossible for both output transistors to be on at one timeand cross over to occur regardless of supply voltage.

The collectors of transistors T1 and T2 form the output of adifferential amplifier.

When a potential difference occurs between the collectors of T1 and T2,either T3 or T4 will be driven into conduction depending on the polarityof the potential difference. The collectors of T3 and T4 are connectedto the bases of t5 and T6 respectively, so that conduction of T3 causesconduction of T5, and conduction of T4 causes conduction of T6. When thedifferential output is balanced, i.e., zero potential difference betweenthe collectors, then both T3 and T4 and therefore T5 and T6 are in theoff condition. The output is summarized by the following table T5 T6 VRlVR2 off on any combination VRl VR2 off off of these is VRl VR2 on offpossible.

Here VRl and VR2 denote the voltages developed across resistors R1 andR2 respectively in FIG. 3.

The error-production and indication system shown in FIG. 4 comprises twophotoconductive cells, a means for balancing out any difference betweenthe cells, a differential amplifier for amplification of a signed errorsignal produced by the cells in response to a deviation off course inthe one direction or the other, and a gate for direction of a signalcorresponding to a particular sign to a corresponding relay or the likefor actuation of an indicator means.

The photo-conductive cells PCI and PC2 are connected in series acrossthe positive and negative lines. The junction of the two cells isconnected to a transistor T1 which is connected in the emitter followermode to create a high input impedance, which reduces damping on thecells.

The output of T1 drives one input line of a differential amplifier, ofan integrated circuit construction. The other input line of thedifferential amplifier is held at a constant potential by the potentialdivider RVl, R1 and R2. Preset control RVl is used to eliminate anydifference between the two cells.

When the compass card is centered there will be no potential differencebetween the two inputs of the differential amplifier. If the cardrotates from the center position then one or the other cell will receivemore light and cause the output potential of T1 to vary. The potentialdifference between the two lines will be amplified by the differentialamplifier.

Depending on the polarity of this potential difference, which in turn isdependent on the direction the compass card is off center, the output ofthe differential amplifier will cause either T2 or T3 to conduct. Thecollectors of T2 and T3 are connected to the bases of T4 and T5respectively. Conduction of T2 causes T4 Deviation from a prescribeddirection will thus cause 5 the appropriate reed switch to vibrate.

We claim:

1. An angular position sensor, which comprises:

a casing;

means defining an indentation in the exterior surface of one end of saidcasing and means defining two indentations in the exterior surface of anopposite end of said casing;

a light source disposed in said means defining said indentation in oneend of said case;

two photosensitive elements, each disposed in a different one of saidmeans defining two indentations in said opposite end of said casing;

a light-polarizing card pivotally mounted in the interior of said casingbetween said light source and said two photosensitive elements, saidcard being mounted to pivot in a plane transverse to the plane of lightemitting from said light source;

means defining an arcuate notch in the periphery of saidlight-polarizing card, said means having two radially extending edgessubtending an angle of at said pivot mounting of said card;

a stop fixed to said casing contiguous to said means defining said notchto abut against said radially extending edges to limit the rotation ofsaid card to a permanent magnet affixed to said light-polarizing cardwhich causes said card to rotate in response to the force exerted onsaid magnet by the earth s magnetic field;

two light-polarizing segments disposed in side by side relationshipbetween said polarizing card and said photosensitive elements, each ofsaid segments being aligned with a different one of said photosensitiveelements and being mounted in fixed positions interior to said case suchthat each of said elements is across the path of light impinging on theone photosensitive element with which it is aligned, and such that thelines of polarization of said two polarizing segments are mutuallyperpendicular and are each at an angle of 45 to the lines ofpolarization of said card when said card is in a mid-position withrespect to said casing; and

a liquid placed in said casing to act as a damper to any erratic motionof said polarizing card,

the magnitude of the output from each of said two photosensitiveelements being substantially equal when said card is in themid-position, and as said card rotates up to 45 either side of themid-position, the output from one of said photosensitive elements variesdown to a minimum value while the output from the other one of saidphotosensitive elements varies up to a maximum value, the outputs fromsaid two photosensitive elements thus indicating the sense of anydeviation of an angular position of said casing from a given positionrelative to the direction of the pull exerted by the earths magneticfield on said magnet, and the magnitude of the deviation if it is notmore than 45.

2. A guidance arrangement as claimed in claim 1 in which thephotosensitive elements are connected for the supply of an error signalto an actuating system for adjusting the motion of the object so thatdeviations from the prescribed course are automatically balanced out byservo action 3. A guidance arrangement as claimed in claim 1 in whichthe actuating system comprises a means for balancing out any differencein the properties of the photosensitive elements, a differenceamplifier, and a gate circuit coupled to the difference amplifier outputand actuated in response to the sign of the difference amplifier outputto cause the corresponding motion of a control member.

4. A guidance arrangement as claimed in claim 1 further comprisingindicator means for giving an audio indication of the sign of adeviation off course.

5. An angular position sensor substantially as set forth in claim 1,further comprising:

two oppositely disposed arcuate notches on the periphery of saidpolarizing card, each of said notches substending an angle of 90 andhaving radially extending edges; and

two stops fixed to said casing each of said stops contiguous to adifferent one of said arcuate notches and adapted to abut against saidradially extending edges of said arcuate notches to limit the rotationof said card.

6. An angular position sensor as defined in claim 1 wherein:

said light-polarizing segments are quadrants of circles; and I meansdefining an air space in said casing to provide space into which saidliquid may be displaced upon expansion of said liquid.

7. An angular position sensor as defined in claim 1 including:

program means operatively associated with said light-polarizing segmentswhich position said segments in accordance to a pre-determined requiredcourse at a vessel to which said sensor is associated is to follow; and

signal means associated with said sensor which gives a measure of thedeviation of said vessel from said pre-determined required course.

1. An angular position sensor, which comprises: a casing; means definingan indentation in the exterior surface of one end of said casing andmeans defining two indentations in the exterior surface of an oppositeend of said casing; a light source disposed in said means defining saidindentation in one end of said case; two photosensitive elements, eachdisposed in a different one of said means defining two indentations insaid opposite end of said casing; a light-polarizing card pivotallymounted in the interior of said casing between said light source andsaid two photosensitive elements, said card being mounted to pivot in aplane transverse to the plane of light emitting from said light source;means defining an arcuate notch in the periphery of said lightpolarizingcard, said means having two radially extending edges subtending an angleof 90* at said pivot mounting of said card; a stop fixed to said casingcontiguous to said means defining said notch to abut against saidradially extending edges to limit the rotation of said card to 90*; apermanent magnet affixed to said light-polarizing card which causes saidcard to rotate in response to the force exerted on said magnet by theearth''s magnetic field; two light-polarizing segments disposed in sideby side relationship between said polarizing card aNd saidphotosensitive elements, each of said segments being aligned with adifferent one of said photosensitive elements and being mounted in fixedpositions interior to said case such that each of said elements isacross the path of light impinging on the one photosensitive elementwith which it is aligned, and such that the lines of polarization ofsaid two polarizing segments are mutually perpendicular and are each atan angle of 45* to the lines of polarization of said card when said cardis in a mid-position with respect to said casing; and a liquid placed insaid casing to act as a damper to any erratic motion of said polarizingcard, the magnitude of the output from each of said two photosensitiveelements being substantially equal when said card is in themid-position, and as said card rotates up to 45* either side of themid-position, the output from one of said photosensitive elements variesdown to a minimum value while the output from the other one of saidphotosensitive elements varies up to a maximum value, the outputs fromsaid two photosensitive elements thus indicating the sense of anydeviation of an angular position of said casing from a given positionrelative to the direction of the pull exerted by the earth''s magneticfield on said magnet, and the magnitude of the deviation if it is notmore than 45*.
 2. A guidance arrangement as claimed in claim 1 in whichthe photosensitive elements are connected for the supply of an errorsignal to an actuating system for adjusting the motion of the object sothat deviations from the prescribed course are automatically balancedout by servo action.
 3. A guidance arrangement as claimed in claim 1 inwhich the actuating system comprises a means for balancing out anydifference in the properties of the photosensitive elements, adifference amplifier, and a gate circuit coupled to the differenceamplifier output and actuated in response to the sign of the differenceamplifier output to cause the corresponding motion of a control member.4. A guidance arrangement as claimed in claim 1 further comprisingindicator means for giving an audio indication of the sign of adeviation off course.
 5. An angular position sensor substantially as setforth in claim 1, further comprising: two oppositely disposed arcuatenotches on the periphery of said polarizing card, each of said notchessubstending an angle of 90* and having radially extending edges; and twostops fixed to said casing each of said stops contiguous to a differentone of said arcuate notches and adapted to abut against said radiallyextending edges of said arcuate notches to limit the rotation of saidcard.
 6. An angular position sensor as defined in claim 1, wherein: saidlight-polarizing segments are quadrants of circles; and means definingan air space in said casing to provide space into which said liquid maybe displaced upon expansion of said liquid.
 7. An angular positionsensor as defined in claim 1, including: program means operativelyassociated with said light-polarizing segments which position saidsegments in accordance to a pre-determined required course at a vesselto which said sensor is associated is to follow; and signal meansassociated with said sensor which gives a measure of the deviation ofsaid vessel from said pre-determined required course.